Plaster of paris and method of making the same



- ydrous or semi-hydrous condition.

Patented Jan. 19, 1926.

UNITED STATES PATENT orncs- ELLEBY LEWIS WILSON, OF raovrnnncnnnonn ISLAND, ASSIGNOR '10 BUMFOBD CHEMICAL WORKS, 013 PROVIDENCE, RHODE ISLAND. A CORPORATION OF RHODE ISLAND.

No Drawing.

5 certain new and useful Improvements in Plaster of Paris and Methods of Making the Same, of which the following is a specification.

In the preparation. of plaster of Paris the 10 Characteristic step by which gypsum or calcium sulphate is transformed into its dehydrated state is generally referred to as calcination.

My invention has to do with the manu- 15 facture of plaster of Paris from either byroduct or mineral sulphate of lime in its Byproduct calcium sulphate results from reactions between calcium compounds and sulphuric acid, or by using some form of calcium salt such as the oxide, hydrate or carbonate as an agent for neutralizing excess sulphuric acid in sulphonatin or other processes where neutralization 0 acid 1s essential or necessary.

The resultant of either process is a finely divided precipitate of calcium sulphate crystals, which in general ractice usually occur in the hydrous form oi crystallization. It has been recognized that this precipitated sul hate of lime was of considerable poten-' tia value if it could be converted into a high grade plaster of Paris by economical methods.

' Up to the present time it has been impossible to secure a high grade, uniformly calcined plaster of Paris from such by-product calcium sulphate, that would pass the.

standard specifications of strength necessary for use in the building trade industry.

-Part of this difficulty at least has been due to the physical condition of this procipitated calcium sulphate'resulting from the method of its production. This generally-leavesa crystalline calcium sulphate residue in a finely divided or almost unitary crystal form, having a sticky or pasty consistency, in which, condition expensive methods of treatment arere uired in order to properly prepare it for ca cination.

The calclnation of hydrous calcium sulphate despite its apparent simplicity ap-v pears to have been imperfectly analyzed and PLASTER OF PARIS AND METHOD OF MAKING THE Application filed August 22, 1924. Serial No. 733,550.

comprehended. It has remained more a matter of rule of thumb operation or of.

judgment drawn from experience rather than a result of any scientific conception. For example, the'desirability of complete and uniform calcination has been recognized but generally has been found unattainable without over-calcination of some; parts of the batch. Over-caloination of any part has been recognized as undesirable but is generally only avoidable at the expense of incomplete calcination of a portion of the bate My invention contemplates the treatment of this precipitated calcium sulphate by the compacting of the same through a process of accretion, so as tosecure the presence of a maximum number of particles in a given space, and in subsequently converting the compacted mass to an interstitially extended state in which it has a maximum of entrained air and is mechanically free for manipulament of such by-products by which their original defects are overcome, I would point out that we may have, either as the direct result of such a process or readily obtain from any other related process, calcium sulphate in a state of slurry, as distinguished from a dry comminuted state, which is the general condition from the resent practice when operating on a minera gypsum.

In the case of such a slurry produced from-a previous wet process or from an actu'al wet grinding of the mineral gypsum into a'slurry, I concentrate the slurry preferably by mechanical manipulation, as for exam 1e, by subjecting it to the action of centrifugal force. Where this slurry occurs as a by-product there is very certain to be present certain deleterious elements of yarious characters which have generally a weakening 'efiect upon the setof the plaster.

This is particularly true of slurries resulting from acid or alkaline reactions, as for example, in the production of phosphoric acid. In connectlon with such a manufacture it has been found by a recent independent invention not my own that it is possible to neutralize the slurry to form salts which geous case for the application of the b force the excess moisture in the slurry may are either nondeleterious or which may be remove with the excess water as a liquor. uch a process offers an extremely advantaprinciples of my invention.

As pointed out above my invention contemplates a crystal accretion and I find that this can be accomplished simultaneously with the separation of the it nor by simply making that separation a violent physical expulsion as by subjecting the slurry to the action of centrifugal force. Under such be reduced to or less'and at the same time the original finely divided crystalline I calcium sulphate becomes densified throughout by a crystal accretion in which there seems tobe an increase of specific density and a rearrangement and compacting or consolidation of crystals or groups of crystals into larger oups. This produces a ve characteristics of the resultant plaster of Paris.

Calcium sulphate crystals in the usual slurry usually appear in a highly attenuated form. Under centrifugal action these crystals or crystal groups become augmented b accretion or by the twinning action characteristic of this mineral. Under magnificathan these crystals or crystal groups, densified or compacted by pressure as heretofore f described, show a characteristic crystalline accretion in Whichthe au entation of the crystals as well as the twmning and interlacin of the crystal mass is quite apparent.

In at er words, the pressure causes the crystale to coalesce and while the exact reason for the physical change which takes lace is diflicult of statement, it would seem t at the remarkable crystalline accretion which characterizes my roduct is due to a possible softening of t e surface of the crystals by the compressive action on its water of crystal'lization by which its surface may bond to the surfaces taming cially impracticable to handle,

double bonding and twinning comes permanent upon release of the pressure. 7

As suggested above, calcium sulphate conapproximately 20% free water is apt to have a sticky or pasty consistency which makes it inconvenient or commerespecially in such a process as calcination.

- self a'poor conductor of heat calcination has usually beeneflected only b strenuous mechanical stirring but even with such stirring the calcium sulphate in its usual form has of adjacent crystals, which Being m it:

been resistant to uniform heating. This has resulted in under-calcination of part of the charge and over-calcination or burning of other parts, both of which materially reduce the strength of the resultant plaster.

I therefore take the calcium sulphate after the expulsion of the liquor and convert it into a non-compacted interstitially extended state in which it will have a considerable amount of air. I preferably accomplish this y a shaving or crumbling action in which the mass is reduced to a state in which the mass is so extended that every portion is exposed to the air. In such a state the material handles freely without clogging or compacting. Thus in the calcining kettle it not onlv stirs freely but has initially so much air within its mass that-heat is readily conducted throughout thus making possible the greatest uniformity in the calcming operation.

It is for this reason that by-product calcium sulphate when fed into t e calcining kettles in this extended state in accordance with my invention can be converted into highly uniform plaster of Paris of excellent crystallizing qualities as is evidenced by a ry considerable increase in ultimate strength. discernible c ange 1n some of the physical I u some cases this increase in strength is as high as 50% over the same raw material treated after the old manner of preliminary drying before calcination.

My invention is capable of being variously practiced as to steps and features y which may be employed singly or variously combined in commercial processes. My invention is also capable of practice with various apparatus all without departing rem the spirit of my invention, but I have found that the use of centrifugal force as a means of mechanical expulsion, (of excess liquor or water,) as ugal machine, produces a cake well adapted to the mechanical conversion to a non-compacted state. and in fact such a cake may be shaved from the machine sothat the material is delivered readv for the calcining kettles, thus eliminating the drying process neces sary in present methods of calcination. In discussing and claiming my invention I have been obliged to use certain terms to specify physical changes in the calcium sulphate, and in the plaster. Such terms as talline accretion or augmentation, coalescense and re'gelation are terms eminvention may doubtlem be found useful in applied in the centrifother processes and in the production of other materials. All such is to be considered as within the scope of my invention if within thelimits of the appended claims.

What I therefore claim and desire to secure by Letters Patent is: 1. In the reduction of plaster of Paris from reciprtated calcium sulphate, that step w ich consists in, efi'ecting crystalline accretion by subjating the material to pressure to cause coalescence of the crystals.

2. In the production of plaster of Paris a from finely divided precipitated gypsum,

that step which consists in effecting twinnin of the finer individual crystals into agg omerated crystal groups through pressure.

3. The process of producing plaster of Paris from precipitated gypsum, which con- 20 sists 'in effecting ctr-ystalline accretion of the crystals so that ey will coalesce, in con- .crysta s to coalesce,

verting the coalesced crystals into a noncompacted, interstitially extended mass upon release of the pressure, and in uniformly calcining the extended mass.

4:. The process of producing plaster of Paris from precipitated gypsum, which consists in effecting crystalline accretion by sub-. jectin the material to pressureto cause the in converting the coalesce crystals into a non-compacted, interstitially extended mass, and 1n uniformly calcining the extended mass.

5. As a new material of increased tensile Q strength for use in the manufacture of plaster of Paris, egrecipitated calcium sulphate characteriz by a compacting of its finer individual crystals into agglomerated crystal groups.

In testimony whereof I aflix my signature.

ELLERY LEWIS WILSON. 

